In the past, the principal means of measuring flow in a gas or air stream was with the use of a single pitot having two pressure taps--one for measuring static head and one for measuring total static and velocity head. Such an arrangement only effectively indicated the velocity at one point in the flow stream. To obtain a more meaningful flow with such a pitot tube, it was necessary to scan the flow stream with the pitot. To scan the flow stream was difficult in itself, plus it was difficult to properly position the pitot with respect to the moving air or gas of the flow stream. In addition accuracy and effectiveness was influenced by the tendency of the pitot to plug, and the fact that the total system was not designed to be accurately responsive to very low differential pressures produced. Thus, the single pitot was never really effective enough to be widely accepted and used in commercial installations, and hence never enjoyed substantial success in such applications.
More recently, a type of branch averaging pitot system has been employed, such as that disclosed in U.S. Pat. No. 3,685,355. However, this type of air monitoring system has encountered some difficulties, and has never been accurate and precise enough for commercial installations. In particularly, the openings in the pitots are proned to becoming plugged, and even when only partially plugged, the air monitoring system loses much of its effectiveness.